No matter how much dexterity an actor has, as long as the target has a positive evasion, normal attacks can still miss. If the enemies have an evasion of zero, the attacker needs a dexterity score higher than 8 times the target's agility in order to guarantee a hit. Doing so will also boost critical hit rate to 32% or more. You can give enemies negative evasion scores though, but that will affect the accuracy of all actors.
Skills will always hit if the Hit Rate box is set to 100 and the EVA-F box is set to 0 unless the actor suffers from a status effect that lowers Hit Rate. Of course, if you don't want an actor to ever miss there's no reason to make him/her vulnerable to blindness or whatever you choose to name the status effect.